Field of the Invention
The present invention relates to casein molded articles formed by molding, spinning, casting a film, or the like, and to a method for preparing the same. The casein molded articles of the present invention are useful as food materials, natural protein fibers or films, and the like.
Description of the Background Art
Industrial application of casein which is the major component of milk proteins, has been studied in many fields. Although some of these studies are actually practiced in industries, the amount of casein used in these applications is limited. The reason is that casein is difficult to be molded, because this protein easily forms a stable micelle structure due to its macromolecular surface activity.
Fibers have been manufactured from unmodified milk casein many years ago in Italy and sold under the trademark of LANITAL. This fiber was not favorably accepted because of its inferior mechanical characteristics and the manufacture was stopped. As the structure of casein protein was elucidated, the reasons for the poor mechanical properties were clarified.
Casein protein is a phosphoprotein possessing a macromolecular surface activity which causes the protein form a micelle structure. The micelle structure renders casein stable in milk. When separated from milk, casein will form globular micelles if an alkaline earth metal, such as calcium or magnesium, is present. Such globular micelles are difficult to disperse in a medium and difficult to mold.
There have been several proposals for dispersing casein, however, none brought about good results. For example, Japanese Patent Laid-open (kokai) No. 138145/1987 discloses a method of dissolving caseinate in an ethanol aqueous solution and making films by fluid spreading. However, caseinate becomes too hard when heated in a drying step, preventing it from being formed into fibers or films. This method therefore has not been industrially successful. Journal of Japan Agrichemical Society, 61, 1087-1092 (1987) proposes a method of dissolving or dispersing casein molecules by breaking down the micelle structure in a casein solution. The method involves treating the casein solution with a chelating resin, thereby removing the metals, e.g. calcium, from the casein solution. The report states that after the treatment, the casein molecules form sub-micelles, but does not describe how the sub-micelles can be used.
As mentioned above, the reason that the past trials of making fibers from milk casein have been unsuccessful lies in the molecular structure of the naturally occuring casein protein. Casein protein is a water-insoluble macromolecular surfactant consisting of hydrophobic protein and hydrophilic phosphoric acid groups. The phosphoric acid groups are bonded to counterions, i.e. metals such as calcium and magnesium, which induce the protein to form globular micelles. This configuration makes casein molecules difficult to orient in the longitudinal direction necessary to obtain an acceptable fiber. Obtaining fibers and other molded articles with acceptable mechanical properties from casein, therefore, has not been successful.